In order for a print imaging support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for ‘imaging media’ suffer low consumer acceptance.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements depending upon the mode of image formation onto the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals without which a stain appears on the print border accompanied by a severe loss in image quality. In the formation of ‘photo-quality’ ink jet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. If the ink is not absorbed quickly, the elements block or stick together when stacked against subsequent prints and exhibit smudging and uneven print density. For thermal media, it is important that the support contain an insulative layer in order to maximize the transfer of dye from the donor to produce higher color saturation.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. For example, it is known that a conventional photographic paper comprises a cellulose paper base that has applied thereto a layer of polyolefin resin, typically polyethylene, on each side, which serves to provide waterproofing to the paper and also provides a smooth surface on which the photosensitive layers are formed. In another imaging material, described in U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described therein have a microvoided layer in combination with coextruded layers that contain white pigments, such as TiC2, above and below the microvoided layer. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create ‘photo-quality’ ink jet paper.
In all of the above imaging supports, multiple operations are required to manufacture and assemble all of the individual layers. For example, photographic paper typically requires a paper-making operation followed by a polyethylene extrusion coating operation, or as disclosed in U.S. Pat. No. 5,866,282, a paper-making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging bases.
It is also well known in the art that traditional imaging bases consist of raw paper base. For example, in typical photographic paper as currently made, approximately 75% of the weight of the photographic paper comprises the raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly. Additionally to minimize environmental impact, it is important to reduce the raw paper base content, where possible, without sacrificing the imaging base features that are valued by the customer, i.e., strength, stiffness, and surface properties of the imaging support.
An important corollary of the above is the ability to recycle photographic paper. Current photographic papers cannot be recycled because they are composites of polyethylene and raw paper base and, as such, cannot be recycled using polymer recovery processes or paper recovery processes. A photographic paper that comprises significantly higher contents of polymer lends itself to recycling using polymer recovery processes.
Existing composite color paper structures are typically subject to curl through the manufacturing, finishing, and processing operations. This curl is primarily due to internal stresses that are built into the various layers of the composite structure during manufacturing and drying operations, as well as during storage operations (core-set curl). Additionally, since the different layers of the composite structure exhibit different susceptibility to humidity, the curl of the imaging base changes as a function of the humidity of its immediate environment. There is a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
The stringent and varied requirements of imaging media, therefore, demand a constant evolution of material and processing technology. One such technology known in the art as ‘polymer foams’ has previously found significant application in food and drink containers, packaging, furniture, and appliances. Polymer foams have also been referred to as cellular polymers, foamed plastic, or expanded plastic. Polymer foams are multiple phase systems comprising a solid polymer matrix that is continuous and a gas phase. For example, U.S. Pat. No. 4,832,775 discloses a composite foam/film structure which comprises a polystyrene foam substrate, oriented polypropylene film applied to at least one major surface of the polystyrene foam substrate, and an acrylic adhesive component securing the polypropylene film to the major surface of the polystyrene foam substrate. The foregoing composite foam/film structure can be shaped by conventional processes, such as thermoforming, to provide numerous types of useful articles including cups, bowls, and plates, as well as cartons and containers that exhibit excellent levels of puncture, flex-crack, grease and abrasion resistance, moisture barrier properties, and resiliency.
Foams have also found limited application in imaging media. For example, JP 2839905 B2 discloses a 3-layer structure comprising a foamed polyolefin layer on the image-receiving side, raw paper base, and a polyethylene resin coat on the backside. The foamed resin layer was created by extruding a mixture of 20 weight % titanium dioxide master batch in low density polyethylene, 78 weight % polypropylene, and 2 weight % of Daiblow PE-M20 (AL)NK blowing agent through a T-die. This foamed sheet was then laminated to the paper base using a hot melt adhesive. The disclosure JP 09127648 A highlights a variation of the JP 2839905 B2 structure, in which the resin on the backside of the paper base is foamed, while the image receiving side resin layer is unfoamed. Another variation is a 4-layer structure highlighted in JP 09106038 A. In this, the image receiving resin layer comprises 2 layers, an unfoamed resin layer, which is in contact with the emulsion, and a foamed resin layer, which is adhered to the paper base. There are several problems with this, however. Structures described in the foregoing patents need to use foamed layers as thin as 10 μm to 45 μm, since the foamed resin layers are being used to replace existing resin coated layers to the paper base. The thickness restriction is further needed to maintain the structural integrity of the photographic paper base since the raw paper base is providing the stiffness. It is known by those versed in the art of foaming that it is very difficult to make thin uniform foamed films with substantial reduction in density especially in the thickness range noted above.
A further requirement for a reflective support is to provide a white or near white appearance, as well as an opaque platform for the imaging layer. A white background is necessary to provide the highlight areas of the image. An opaque layer is needed to maximize the scattering of the light and prevent show through.
Traditional photographic prints, as well as ink jet, thermal and all other reflective imaging methods employ a small number or set of subtractive color pigments, dyes, or colorants which, when used by themselves or in any combination, can result in the entire gamut of color sensation. When all of these are present at their maximum in the imaging layer of the support, the observer sees a black appearance. When they are used in uneven ratios, any color in color space can be reproduced for the observer. When they are all absent from the imaging layer of the support and one is looking through that layer to the support below, a white appearance is seen by the observer. Therefore, the reflective image can only be as white and as bright as is the support layer below. Great pains are taken in the selection of materials, their concentration, combination and/or placement in a reflective imaging support so that the appearance of whiteness can be maximized.
Since the perception of whiteness varies between observers, colorants are added to the reflective imaging support, as needed. In general, studies have shown that most observers prefer a slightly blue white as opposed to a true white or slightly yellow white and, hence, the inclusion of colorants that result in a slight blue hue to the imaging support member. (Principles of Color Technology, Billmeyer and Saltzman, 2nd edition, John Wiley & Sons, New York, 1981, p. 66).
The same considerations of materials selection, combination, concentration, and placement also hold true when one is concerned with the opacity of a reflective support. One wishes to prevent the show through of the reflective image below the one being viewed in a stack of images or the non-white surface that the reflective image is resting on or is mounted to by providing adequate opacity. In the case of resin coated photographic prints, the layer immediately below the emulsion has a large impact on the image sharpness of the print due to the scattering of light during exposure of the print paper to the negative (To RC or Not to RC, Crawford, Gray and Parsons, Journal of Applied Photographic Engineering, 110-117 (1979)). Large amounts of TiO2, in the 10 to 15 percent range or higher, are added to this layer to enhance image sharpness and, in turn, hiding power and opacity of the imaging support. Given the fact that ink jet, thermal, and most high-end imaging media were derived from and are now in competition with photographic imaging media, the need for comparable degrees of opacity becomes necessary.
In is further known in the art to provide a voided polyester film with a non-continuous voided core density for use in labels. U.S. Pat. No 5,084,334 discloses a void containing polyester film using non-compatible blends of polyester resins in which voids are formed around fine particles. In order to create voids, the sheet is drawn in both the machine and cross machine directions. This film requires the dispersion of particles sizes in a manner that provides a concentration of finer particle near the surfaces and a larger particle size near the central part of the polyester core in order to provide a different amount of voiding when biaxially stretched at the surface as opposed to the core. It should be noted that this is material depends on a particle to provide voiding.
In is also known in the art to make polystyrene foam with a high-density skin layer. Johnson et al in U.S. Pat. Nos. 4,456,571 and 4,098,941 discloses polystyrene foam that is extruded upwardly as a tube into a cooling media of boiling water, both the interior and exterior surfaces of the cylindrically shaped extrudate passing through a bath of boiling water of variable depth. The polystyrene foam extrudate produced by such a method has a high density at the interior and exterior surfaces, with the density progressively decreasing from these surfaces toward the center core of the extrudate. U.S. Pat. Nos. 4,456,571 and 4,098,941 also disclose a method and apparatus for producing a foamed polymeric sheet having comparatively high-density skin layers. Molten polymer containing a foaming agent is extruded from a die into a post-extrusion region defined by the die and a spaced pair of rotating chill rolls, which are spaced from the die. The post-extrusion region is maintained at a pressure sufficient to at least inhibit expansion of the foamable molten polymer through the use of cooled sealing elements, which occupy a substantial portion of the space between the die and the chill rolls, and are spaced from the chill rolls. The rolls and the sealing elements are cooled so that foamable molten polymer in the space between the sealing elements and the rolls becomes solidified, completing the seal. The chill rolls are maintained at a temperature below the temperature of the molten polymer to aid the skin formation on the surfaces of the polymer. If desired, the polymer can pass directly from the chill rolls into a water bath. It would be desirable to have a stiff foamed polyolefins, specifically polypropylene, sheets or films that have a gradient or variable density from the center of the core to the surface without having to provide a skin layer.
It is known to produce polystyrene foam structures that have a comparatively low core density in relation to comparatively high skin layer densities. See, for example, U.S. Pat. Nos. 3,864,444 and 3,299,192.
U.S. Pat. No. 3,299,192 states that the rigidity, liquid handling, and thermal insulation capability of foamed plastic pipe is enhanced by quench chilling the internal and external walls of a tube within a short time after it emerges from an extrusion die. The patent notes that such chilling produces an impervious and non-porous internal and external skin on the pipe. The present invention provides an imaging media comprising a foam core with increasing density from the center to the outer surfaces which is then coated with polymer flange layers or laminated with polymer flange layers.
U.S. Pat. No 6,077,065 describes a process for making non-foamed smooth surfaced solid plastic films. Such a device helps to minimize air pockets by providing a compliant roller. While this process provides improvements in the area of solid films such a process does not yield foam films of sufficient smoothness for imaging purposes. It would be desirable to make smoother surface foamed film.
U.S. Pat. No. 5,674,442 provides a solid film with improved optical properties such as gloss by extruding a melt polymer into a nip formed by a chill roller and an endless moving belt. While such a device is useful in making smooth surfaced solid films on both sides simultaneously, utility in making a smooth surface foamed film with control of the foam void size is very limited. It would be desirable to have a process that can form a smooth surface for imaging purposes while controlling the foam void formation to provide a uniform density.
Other patents that address foaming also includes U.S. Pat. No. 5,277,852 for making foamed PVC blown films with a post extrusion pressure zone and U.S. Pat. No. 5,667,740 for making lightweight cellular foamed PVC molded materials. While these processes are useful in their own right, they are not suitable for making imaging materials from foamed polyolefins, specifically polypropylenes.
U.S. Pat. No. 6,514,659 discloses a means of making a foam core for imaging applications that has a smooth surface. In this approach the smoothness of the overall article, preferably an imaging element, is achieved via thickness and modulus of the core and the flange layers. While this approach is useful and provides a smooth imaging element it is very costly and the production rates are slow and difficult to maintain because of flange layer thickness. It would be desirable to make a superior foam core with smooth surface that requires less polymer flange thickness. U.S. Pat. Nos. 6,566,033, 6,537,656, 6,447,976 and U.S. patent applications 20030219663A1, 20030152760A1 also describe foam core elements for use in imaging applications, but do not discuss the use of density gradients.